There are many types of well-known food cooking ovens which use gas combination, microwaves, or infra-red heat sources and some which disclose a combination thereof useful for cooking deep frozen, refrigerated or pre-cooked food. One example is found in U.S. Pat. No. 4,999,468.
While microwave heating sources are useful to quickly heat food, there are disadvantages for using such a source in a combination, progressive heating source oven. One disadvantage is the requirement of shielding the microwave energy which requires a special structure to isolate such a heating station from the remaining heating station or stations and the surrounding environment while permitting access in and out of the shielded area. Further, metal food pans in which a product is cooked and delivered pose a problem in a microwave oven. Additionally, microwave energy tends to heat the food product in a less uniform manner and therefore a substantial portion of the heat energy is transferred to the food by conduction. Conventional infra-red sources needed to brown the outside of a food product operate in a similar manner relative to requiring conduction to heat the inside of the product, however, are useful to brown or crisp the outside.
The construction of an efficient microwave oven requires the use of wave guides or resonant chambers and often moving parts to more uniformly apply heat energy. These factors further complicate the construction of a more simple and less expensive multiple, progressive heat source oven structure and the manner in which such may be conveyed between multiple heating stations.
As applied to an automated food cooking dispenser for pizza for example, wherein a frozen or refrigerated pizza type food must be fully and properly cooked as quickly as possible, it is desirable to initially heat the food portion uniformly as fast as possible and then subject it to an infra-red source for browning or crisping the outside of the food. While microwave energy has been suggested for the initial heating and cooking stage, its disadvantages make it less than fully satisfactory to construct and operate in connection with a second or final stage infra-red cooking source to obtain an efficiently cooked food portion in the shortest practical time.
Prior to the present invention, radio-frequency energy has long been used for certain industrial heating applications, however, it has not been used in the cooking of foods or in combination with infra-red sources to construct a fast and efficient multiple stage or progressive food cooking oven. Radio frequency heating as used herein refers to the type wherein a pair of spaced electrodes having a dielectric material between is employed to form a capacitor through which a uniform high frequency field is passed to cause a molecular action to take place through out the cross-section of the dielectric material. It specifically does not include micro-wave heating as it is generally known and used in conventional ovens which employ relative higher frequencies and shorter wave lengths to attempt to efficiently heat the product.